The invention relates to a drive unit for safety belt tensioners which have a drive which can be triggered on the response of an acceleration sensor and which can be rotationally fixedly connected to the belt winding reel. The drive is located in a drive chamber which is formed by two plates, which extend parallel to one another and which are connected to one another, and which has a drive band of which at least one end is fastened to a drive shaft. On the response of the acceleration sensor, the drive band is exposed from one side to an expanding gas coming from a gas generator such that the drive band unwinds and thereby drives the drive shaft. Drive units of this kind are generally known (cf. DE-A-199 61 109.2).
The performance of such a drive unit depends, among other things, on the amount of the pressure which builds up through the expanding gas inside the space which is formed by the plates and a loop formed by the drive band. The pressure building up is, however, reduced by gas which escapes via the interface drive band/plates.
A possibility to improve the quality of the gas seal between the drive band and the plates lies in the fact of using a two-ply drive band which is provided with an additional rubber-like sealing belt which is disposed between the plies of the drive band and which projects a little at the edges of the drive band and in this way provides a more effective sealing effect of the drive band and thus a reduced gas loss via the interface drive band/plates.
The disadvantage of such an apparatus is, however, the relatively complex manufacture of the multi-ply drive bands. An optimum sealing effect can only be achieved with an exact parallelism and absolutely constant width of the three belts. The low tolerances required for an effective sealing effect of the drive band make the production of such drive band complex and expensive.